The present invention relates in particular to a method for effecting bending of a workpiece plate by way of causing a rear-edge of the workpiece plate to bump against stop members of a back gauge unit and an apparatus for effecting the method in such a technical field applicable to a bending apparatus such as a press brake incorporating a back gauge unit.
Conventionally, a bending apparatus called a press brake is used for effecting bending of a workpiece plate such as a metal plate.
As shown in FIG. 6 and FIG. 7, a conventional press brake comprises a main press body 1 and a back gauge unit 2 disposed at the back of the main press body 1. In the main press body 1, a lower table 4 for mounting a lower mold 3 thereon and an upper table 6 for securing an upper mold 5 thereto are opposedly disposed above and below the front surface of the main press body 1. The upper table 6 is vertically movable via an interlinked drive mechanism such as an oil-pressure cylinder not shown. While the upper table 6 descends, a workpiece plate 8 disposed on the lower mold 3 is pressed into a V-shaped channel 7 of the lower mold 3 by the upper mold 5. Thus, the workpiece plate 8 is folded at a predetermined angle.
The above-referred back gauge unit 2 functions for positioning the workpiece plate 8 on the lower mold 3 in correspondence with bending degree predetermined for the workpiece plate 8. A pair of stop members 9 and 10, disposed to the left and to the right, are respectively movable in the front/rear direction A (in the depthwise direction of the main press body 1), in the horizontal direction E (in the widthwise direction of the main press body 1) and in the vertical direction C (in the height direction of the main press body 1).
The reference numeral 11, shown in FIG. 7, designates a slide guide for reciprocatably and slidably supporting the above stop members 9 and 10 in the horizontal direction E, where both ends of the slide guide 11 are respectively linked with a pair of drive mechanism 12 and 13 such as a ball screw mechanism.
The drive mechanisms 12 and 13 are respectively provided with independent drive sources such as servo motors. By operating the drive mechanism 12,13 individually to cause both ends of the slide guide 11 to be shifted in the front/rear direction, positions of the stop members 9,10 and a gradient angle .alpha. in a horizontal plane, defined by a front of a stop surface X, can respectively be set. The stop surface X corresponds to a surface against which a workpiece plate 8 is placed in contact. In other words, this is a virtual perpendicular plane formed by tips of the stop members 9 and 10. If the stop members were of single number, a work-supporting surface at a front edge surface of the stop member becomes the stop surface X. The gradient angle .alpha. specifies such a direction for causing the stop surface X to bump against a r ear edge of the workpiece plate 8. Gradient of line segment formed by the stop surface X on the vertical horizontal surface in contact with the stop surface X corresponds to the gradient angle .alpha..
The stop members 9 and 10 respectively have front-edge surfaces each forming a flat work-supporting surface 14. Initially, an operator securely holds a front edge portion of a workpiece plate 8 and then inserts a rear edge portion between the upper mold 5 and the lower mold 3 of the main press body 1 to cause the rear edge portion of the workpiece plate 8 to bump against work-supporting surfaces 14 of the respective stop members 9 and 10. Next, the main press body 1 is operated via a pedal means so that the workpiece plate 8 is bent.
In the case of processing a workpiece plate 8 into a folded state shown in FIG. 8, initially, reference target value B of a dimension between a rear edge 15 and a bending position 16 (this is called the "bending dimension") and reference target value .theta. of a bending angle for each workpiece plate 8 are respectively set. Next, the workpiece plate 8 is subject to a bending process in order that bent angles .theta..sub.L and .theta..sub.R at both end portions can correctly match the reference target value .theta. and bent dimensions B.sub.L and B.sub.R at both end portions also correspond to the reference target value B.
As shown in FIG. 9, the bending dimension is determined by a distance S between a blade tip 5b of the upper mold 5 and respective stop members 9 and 10 of the back gauge mechanism. On the other hand, as shown in FIG. 10, the bending angle is determined by input amount d generated by insertion of the upper mold 5 into the V-shaped channel of the lower mold 3, and thus, the distance S and the input amount d are respectively set in correspondence with the target values B and .theta. predetermined for the bending dimension and the bending angle.
The above referenced bending dimension is expressed by means of distance between contact position of the blade tip 5b of the upper mold 5 being abutted to the workpiece plate 8 and the rear edge 15 of the workpiece plate 8 (this is called "absolute dimension"). Further, the bending dimension is also expressed by means of external dimension B1 corresponding to distance between the rear edge 15 of the workpiece plate 8 and folded external surface 8a or by means of internal dimension B2 corresponding to distance between the rear edge 15 of the workpiece plate 8 and folded internal surface 8b. A difference between the external dimension B1 and the internal dimension B2 corresponds to the thickness of the workpiece plate 8. Generally, the external dimension B1 is greater than the absolute dimension, whereas the internal dimension B2 is smaller than the absolute dimension. However, in such a case in which the folded corner portion has a large circular arc, the internal dimension B2 may be greater than the absolute dimension.
When bending a workpiece plate 8 by operating a press brake incorporating the above structure, initially, a pair of drive mechanism 12 and 13 on both sides of the back gauge mechanism 2 are discretely driven to shift a pair of stop members 9 and 10 in the front/rear direction A, and the stop members 9 and 10 are respectively positioned to a predetermined position corresponding to target value B of the dimension predetermined for the workpiece plate 8. In the next step, the workpiece plate 8 is inserted between the upper mold 5 and the lower mold 3 to cause the rear edge 15 of the workpiece plate 8 to bump against the work-supporting flat surfaces 14 of respective stop members 9 and 10. While holding the above condition, the workpiece plate 8 is treated with a bending process.
When the stop members 9 and 10 disposed on both sides are positioned to the identical spots in the front/rear direction A, theoretically, bending dimensions B.sub.L and B.sub.R at both edge portions of the workpiece plate 8 should coincide with each other. However, if precision of the upper mold 5 is not maintained along a full length, then bending dimensions B.sub.L and B.sub.R will not be identical to each other.
The reference numeral 5a shown in FIG. 11 designates a reference surface for mounting the upper mold 5, where a relative positional relationship between the upper mold 5 and an upper table 6 is specified. When the upper mold 5 is secured to a bottom portion of the upper table 6, according to the reference surface 5a, the blade tip 5b of the upper mold 5 is positioned on a central line C of the upper table 6. Since the blade tip 5b of the upper mold 5 actually bends the workpiece plate 8, precision is required for positional relationship between the reference surface 5a for mounting the upper mold 5 and the blade tip 5b. Nevertheless, if precision of the positional relationship between the reference surface 5a for mounting the upper mold 5 and the blade tip 5b is not maintained along the full length of the upper mold 5, then bending dimensions B.sub.L and B.sub.R at both ends of the workpiece plate 8 will not coincide with each other.
Likewise, the bending dimensions B.sub.L and B.sub.R at both ends of the workpiece plate 8 do not coincide with each other in the case in which precision of positional relationship between the reference surface 5a shown in FIG. 11 and the back gauge unit 2 is not maintained along the full length.
Further, the back gauge unit 2 generally uses ball-screw drives functioning as the drive mechanism 12 and 13 for positioning the stop members 9 and 10 on both sides thereof, so if there is any difference in precision or performance characteristic of the ball-screw means on both sides, the bending dimensions B.sub.L /B.sub.R at both edges of the workpiece plate 8 do not coincide with each other.
Further, if surfaces for supporting respective stop members 9 and 10 secured to the slide guide 11 are devoid of a precise finish, the stop members 9 and 10 cannot be positioned with satisfactory precision. Then, like the above case, bending dimensions B.sub.L /B.sub.R at both edges of the workpiece plate 8 do not coincide with each other.
Since bending dimensions B.sub.L and B.sub.R at both edges of the workpiece plate 8 do not coincide with each other whenever erroneous factors cited above are generated, initially, a trial bending operation is performed on a workpiece plate 8, and then bending dimensions are measured at both edge portions. If measured values are not coincident with target values, then the operator discretely controls front/rear directional positions of respective stop members 9 and 10 by operating drive mechanism 12 and 13 on both sides. If measured values of bending dimensions on both sides are not coincident with each other, then operator will optimally shift positions of the stop members 9 and 10 in the front/rear directions A, thereby adjusting the gradient angle .alpha. of the horizontal surface of the front of stop surface X formed by tip portions of the stop members 9 and 10 disposed both sides.
Nevertheless, when positions of the stop members 9 and 10 on both sides are adjusted solely based on the operator's own sense, the operator incurs a greater burden from routines than is desired. Unless the operator is quite experienced, extreme difficulty is involved in execution of the above position adjustment work.
Since a satisfactory result cannot be achieved via a single adjustment, the operator is obliged to follow up with repeated adjustment operations in trial and error fashion, thus preventing improvement in efficiency of work and productivity.